The Physics of RHIC Peter Jacobs Lawrence Berkeley National Laboratory

Transcription

1 The Physics of RHIC Peter Jacobs Lawrence Berkeley National Laboratory Why collide nuclei at high energy? RHIC: machine and experiments Physics from the first year of RHIC Outlook SLAC, Nov 13, 2001 The Physics of RHIC 1

2 Schematic Phase Diagram of Strongly temperature early universe Interacting Matter e.g. two massless flavours (Rajagopal and Wilczek, hep-ph/ ) neutron stars baryochemical potential T>>Λ QCD : weak coupling deconfined phase (Quark Gluon Plasma) T< Λ QCD : strong coupling confinement phase transition at T~ Λ QCD? Similar arguments for squeezing cold matter (increasing µ B ) SLAC, Nov 13, 2001 The Physics of RHIC 2

3 Lattice QCD at Finite Temperature Coincident transitions: deconfinement and chiral symmetry restoration Currently only for µ B =0 (but some recent developments ) Ideal gas (Stefan- Boltzmann limit) F. Karsch, hep-ph/ Critical energy density: 4 ( 6 ± 2) TC T C ~ 175 MeV ε C ~ 1 GeV/fm 3 SLAC, Nov 13, 2001 The Physics of RHIC 3 ε C =

4 Order of the Phase Transition Only partially understood: Three massless flavours: first order Two massless flavours: second order Two light and one heavy: probably second order µ B =0, physical strange quark mass: rapid cross over? So what? Early universe (t~10-5 sec): strong first order transition may have generated: primordial black holes strange quark nuggets baryon asymmetries implications for nucleosynthesis SLAC, Nov 13, 2001 The Physics of RHIC 4

5 Can we study the QCD Phase Diagram in the Laboratory? Space-time Evolution of Heavy Ion Collisions Kinetic freezeout: elastic scattering stops Chemical freezeout: inelastic scattering stops SLAC, Nov 13, 2001 The Physics of RHIC 5

6 Observables of the QGP in Nuclear Collisions Nuclear collisions are highly dynamic, no first-principles theory Some tools to distinguish deconfined QGP from dense hadron gas: Direct observation of deconfinement: suppression of J/ψ High energy density: interaction of jets with medium High temperature: direct photons Non-hadronic degrees of freedom: charge fluctuations Quasi-equilibrium at early stage: flow Rapid equilibration, mass shifts: strangeness enhancement Threshold behaviour: must be able to turn effects off s, centrality of collision, mass of system Smoking gun? More likely scenario: QGP is most reasonable picture from many different observables simultaneously SLAC, Nov 13, 2001 The Physics of RHIC 6

7 Digression to the SPS fixed target: Charmonium Suppression in Pb+Pb Matsui and Satz 86: color Debye screening in plasma prevents formation of c-cbar resonances Absorption in cold nuclear matter ΝΑ50 (Pb+Pb, s NN =17 GeV) Β µµ σ(j/ψ)/σ(drell-yan) changes in slope: energy density threshold? successive melting of ψ, χ c, J/ψ Anomalous absorption for central collisions (high energy density) peripheral central SLAC, Nov 13, 2001 The Physics of RHIC 7

8 Charmonium Suppression: compare to models Hadronic models: cold nuclear + comover dissociation QGP models: energy density thresholds + E T fluctuations Also: Capella et al. thresholds and high E T behavior favour QGP models SLAC, Nov 13, 2001 The Physics of RHIC 8

9 Summary of Pb+Pb Collisions at the SPS hadronthermodynamics: Baryon-rich system at y~0 high initial energy density (ε~3 GeV/fm 3?) early equilibration? low mass enhancement of di-electrons: chiral symmetry restoration? direct photons beyond hadronic sources: radiation from plasma? multistrange baryon enhancement charmonium suppression difficult to explain by dense hadronic gas Evidence for deconfinement at the SPS is suggestive but not definitive: theoretical ambiguities: dense hadron gas vs QGP effects if deconfinement indeed seen at top SPS energy, how to turn it off? ( lower energy running) SLAC, Nov 13, 2001 The Physics of RHIC 9

10 The Relativistic Heavy Ion Collider at Brookhaven National Laboratory Dedicated collider for heavy ion physics: Au+Au up to s NN =200 GeV (SPS: s NN = GeV) (polarized) p+p up to s = 450 GeV great flexibility in beams and energies: extensive reference data What is new relative to fixed target experiments? higher initial energy density longer-lived hot phase (much) lower baryon density new physics channels: jets, B-production, much higher statistics: more detailed studies First physics run June-Aug 00: Au+Au at s NN =130 GeV SLAC, Nov 13, 2001 The Physics of RHIC 10

11 Gold Ion Collisions in RHIC PHOBOS 10:00 o clock 12:00 o clock BRAHMS 2:00 o clock PHENIX 8:00 o clock RHIC STAR 6:00 o clock 4:00 o clock U-line Pol. Proton Source BAF (NASA) µ g-2 LINAC High Int. Proton Source BOOSTER AGS 9 GeV/u Q = +79 HEP/NP Design Parameters: Beam Energy = 100 GeV/u No. Bunches = 57 No. Ions /Bunch = T store = 10 hours L ave = cm -2 sec -1 Year 1 Performance: γ = 70 s NN = 130 GeV L = 2 x cm -2 s -1 Integrated L ~ few µb -1 1 MeV/u Q = +32 TANDEMS Oct. 01: 2x10 26 (design) achieved SLAC, Nov 13, 2001 The Physics of RHIC 11

12 The Two Large Detectors STAR Solenoidal field, large-ω tracking TPC s, Si-vertex tracking RICH, TOF, large EM Cal ~420 participants Coils Magnet PHENIX Axial field, high resolution & rates 2 central arms, 2 forward muon arms TEC, RICH, EM Cal, Si, TOF, µ-id ~450 participants E-M Calorimeter Projection Chamber Time of Flight SLAC, Nov 13, 2001 The Physics of RHIC 12

13 The Two Small Detectors BRAHMS 2 conventional spectrometers full phase space coverage Magnets, TPCs, TOF, RICH ~40 participants PHOBOS Table-top 2-arm spectrometer full phase space multiplicity measurement Magnet, Si µ-strips, Si multiplicity rings, TOF ~80 participants SLAC, Nov 13, 2001 The Physics of RHIC 13

14 STAR High Multiplicity Au+Au Collision at s NN =130 GeV colors ~ momentum: low high SLAC, Nov 13, 2001 The Physics of RHIC 14

15 Particle Identification in Heavy Ion Events PHENIX TOF STAR TPC Ξ + K *0 π 0 (PHENIX EMC) SLAC, Nov 13, 2001 The Physics of RHIC 15

16 Digression: ultra-peripheral collisions γγ, γ-pomeron interactions Signature: back-to-back opposite charges Au+Au Au+Au + ρ 0 STAR preliminary STAR preliminary SLAC, Nov 13, 2001 The Physics of RHIC 16

17 Geometry of Heavy Ion Collisions Non-central Collisions Central Collisions z x y Reaction plane Nparticipant: number of incoming nucleons (participants) in the overlap region Nbinary: number of equivalent inelastic nucleon-nucleon collisions SLAC, Nov 13, 2001 The Physics of RHIC 17

18 Experimental Determination of Geometry 3DGGOHV%%& ='& $X $X ='&Ã 3DGGOHV%%& &HQWUDOÃ0XOWLSOLFLW\Ã 'HWHFWRUV Paddle signal (a.u.) STAR 5% Central SLAC, Nov 13, 2001 The Physics of RHIC 18

19 Baryon Density at Midrapidity pbar/p vs s, central collisions of heavy nuclei All four experiments: pbar/p ~ STAR: Λbar/Λ ~ 0.7 Ξbar/ Ξ ~ 0.8 Approaching baryon-free environment But net baryon number still finite (baryon transport over y~5.5) SLAC, Nov 13, 2001 The Physics of RHIC 19

20 Charged particle production (η=0) Central s NN =130: world average dn ch /dη =584±18 central collisions PRL 85 (2000) % increase relative to p+pbar: Au+Au is not a simple superposition grows faster with s than p+pbar: onset of hard scattering? (~Nbinary) SLAC, Nov 13, 2001 The Physics of RHIC 20

21 Particle Production vs. Collision Centrality Rapidity dependence PHOBOS: nucl-ex/ h - <p T >vs dn h- /dη dn ch /dη 0-3% 15-20% pbar-p pp central Pb+Pb NA % Total charged particles for central collisions: 4200±470! η Mean transverse momentum: spectrum much harder than for pbar-p, Pb+Pb@ SPS harder for more central collisions SLAC, Nov 13, 2001 The Physics of RHIC 21

22 Sensitive to gluon saturation? High density QCD: dn = cn part xg( x, Qs ); Qs ~ A dη vary impact parameter vary Q s2 visible in total multiplicity? dn ch /dη/(n part /2) PHENIX PRL 86, 3500 N part Data agree with both simple Glauber (hard/soft eikonal calculation) and high density QCD no discriminating power yet!! SLAC, Nov 13, 2001 The Physics of RHIC 22

23 Bjorken Energy Density Bjorken 83: ideal 1+1 D relativistic hydrodynamics boost invariance η 0 ε= 1 de πr τ dy T πr τ p T 3 2 dnch dη (R ~ A 1/3, τ = 1 fm/c) Central s NN =130: PHENIX E T : ε = 4.6 GeV/fm 3 (nucl-ex/ ) STAR charged particles: ε 4.5 GeV/fm 3 Compare NA49 Pb+Pb@SPS: ε 3 GeV/fm 3 (τ = 1 fm/c) Critical issues: Has equilibrium been achieved? (i.e. hydrodynamics valid?) If so, what is formation time τ? SLAC, Nov 13, 2001 The Physics of RHIC 23

24 p T Spectra: central collisions Kinetic freezeout pbar K - π - Radial flow of matter: common velocity boost stiffer momentum spectrum for more massive particles SLAC, Nov 13, 2001 The Physics of RHIC 24

25 p T Spectra: systematics for p T < 1 GeV/c preliminary Simple hydrodynamics: T apparent = T freezeout + mass * β 2 RadialFlow explosive radial expansion at RHIC high pressure SLAC, Nov 13, 2001 The Physics of RHIC 25

26 Particle Ratios in Central Collisions N. Xu et al. Chemical freezeout Simple thermal model: Partition fn, spectrum of hadrons Parameters T, µ B, µ s, µ I3 Particle ratio (thermal fit) Particle ratio (data) Fit to ratios of antiparticle/particles: π, K, p, Λ, Ξ, K * 0 Typical fits: T ch = 175~200 MeV, µ B ~50 MeV, µ s ~0 Simple model works well: evidence for chemical equilibrium? SLAC, Nov 13, 2001 The Physics of RHIC 26

27 Phase Diagram at Chemical Freezeout Chemical Temperature Tch [MeV] early universe RHIC SPS Lattice QCD thermal freeze-out hadron gas Put parameters from Thermal Model fits on the phase diagram: quark-gluon plasma AGS SIS deconfinement chiral restauration atomic nuclei Baryonic Potential µ B [MeV] P. Braun-Munzinger, nucl-ex/ neutron stars parameters near phase boundary (strangeness) equilibration time for hadronic gas very long (~50 fm/c) do we have more direct evidence of early equilibration? SLAC, Nov 13, 2001 The Physics of RHIC 27

28 Elliptic Flow in Non-central Collisions Coordinate space: initial asymmetry Momentum space: final asymmetry p y multiple collisions (pressure) p x Asymmetry generated early in collision, quenched by expansion observed asymmetry emphasizes early time Second Fourier coefficient v2: v 2 = cos2φ φ = atan SLAC, Nov 13, 2001 The Physics of RHIC 28 p p y x

29 Elliptic Flow (cont d) v2 vs centrality v2 vs p T and particle mass V 2 Hydrodynamic model (PRL 86 (2001) 402) peripheral central N ch /N max Hydrodynamic calculations in reasonable agreement compatible with early equilibration Cascade models require extreme elastic cross sections or huge gluon densities (Molnar and Gyulassy) SLAC, Nov 13, 2001 The Physics of RHIC 29

30 Fly in the Ointment: Pion Interferometry (GGLP/HBT effect) π-π- : y < < pt(gev/c) < STAR, PRL 87, interference of identical bosons at low relative momentum q certain components of q sensitive to duration of particle emission strong 1 st order phase transition large latent heat long stall in expansion long lifetime No variation with s seen explosive expansion?? SLAC, Nov 13, 2001 The Physics of RHIC 30

31 High p T hadrons in Nuclear Collisions leading particle q q leading particle Bjorken, Wang&Gyulassy, Baier et al.: - de/dx: parton traversing medium dissipates energy via brehmsstrahlung - coherence effects: de/dx ~ x 2 - de/dx much larger in deconfined medium than hadronic matter - Full jet reconstruction in Au+Au impossible (underlying event) But also not relevant: observable is softening of fragmentation SLAC, Nov 13, 2001 The Physics of RHIC 31

32 Leading Hadrons in Fixed Target Experiments p+a collisions: σ pa = A α ( ) p t σ pp Central Pb+Pb collisions at SPS Multiple scattering in initial state( Cronin effect ) AA SPS: any parton energy loss effects buried by initial state multiple scattering, transverse radial flow, SLAC, Nov 13, 2001 The Physics of RHIC 32

33 High p T charged hadrons (central collisions) STAR negative hadrons PHENIX vs STAR STAR preliminary UA1 reference data Agreement at ~20% level! SLAC, Nov 13, 2001 The Physics of RHIC 33

34 Ratio STAR/UA1 vs p T Scaling factors: energy dependence, nuclear geometry Hadron suppression in nuclear collisions at high p T STAR preliminary important open issues about systematics of UA1 comparison flavor and isospin are few percent effects SLAC, Nov 13, 2001 The Physics of RHIC 34

35 PHENIX: charged hadrons and π 0 nucl-ex/ , submitted to PRL central normalized to p+p/pbar+p central/peripheral Suppression greater for π 0 than for charged hadrons Suppression larger if reference includes Cronin effect SLAC, Nov 13, 2001 The Physics of RHIC 35

36 PHENIX: π/k/p at high p T Central Au+Au@RHIC very different than ISR: extreme radial flow? exotic fragmentation? baryon junctions? SLAC, Nov 13, 2001 The Physics of RHIC 36

37 Elliptic flow at high p T : predictions Jet propagation through anisotropic matter (non-central collisions) Snellings; Gyulassy, Vitev and Wang (nucl-th/ ) jet jet Finite asymmetry at high p T sensitive to energy density SLAC, Nov 13, 2001 The Physics of RHIC 37

38 STAR: Elliptic flow at high p T STAR Hydro calculation: P. Huovinen watch out for autocorrelation: jets distort reaction plane measurement look at v 1 (1 st order coeff): effect is negligible saturation to 6 GeV: not yet perturbative?? not yet understood SLAC, Nov 13, 2001 The Physics of RHIC 38

39 Elliptical flow vs. inclusive hadron spectra Different views of same physics? Evidence for hadron suppression at high p T Flow or partonic interaction with matter? SLAC, Nov 13, 2001 The Physics of RHIC 39

40 Summary of Au+Au Collisions at RHIC Machine and experiments are working well! Low p T hadrons: low baryon density at y~0 strong evidence for early equilibration high initial pressure and energy density best estimate: ε ~ 4.5 GeV/fm 3 High p T hadrons: suppression of inclusive yields p/π> 1 at p T > 2 GeV: radial flow or exotic fragmentation? elliptic flow persists at high p T Most exciting prospect: jet quenching in dense matter? direct indicator of deconfinement but still a long road: need higher p T, p+p, p+a, γ+jet measurements SLAC, Nov 13, 2001 The Physics of RHIC 40

41 Outlook RHIC Year 2: July-Nov: GeV (design luminosity reached) Dec-Jan: 5 weeks of polarized protons Major detector upgrades in Year 2: STAR: EM calorimeter, inner and forward tracking PHENIX: completed spectrometers, first muon arm Year 2 physics: Very high p T spectra & correlations First charm physics (J/ψ) Many rare probes: detailed dynamics Energy scan: extensive systematics (unique to RHIC) Long term: extensive program of pa, AA, polarized protons, SLAC, Nov 13, 2001 The Physics of RHIC 41